This invention is concerned particularly with packaging machines commonly referred to as vertical form fill and seal (VFFS) machines. It will be described specifically with reference to such machines, but it should be understood that at least certain aspects of this invention are applicable to machines in which the packaging material moves obliquely downwards or even horizontally while bags are being formed. All such machines are encompassed in this context by the expression xe2x80x9cmachines of the type describedxe2x80x9d.
In a VFFS machine, a web of packaging material is drawn along a former and then passes downwards in tubular formation; the edges are sealed longitudinally as the packaging material moves downwards (assuming, as is preferred, that the packaging material moves continuously), after which horizontal seals are made at regular intervals to form individual bags. A measured quantity of product to be packaged is dropped into each bag before a top seal is formed by sealing jaws which simultaneously form the bottom seal of the next bag. The sealing jaws commonly include a cutting device which separates successive bags.
With some materials and in some circumstances, it is desirable to provide a stripping device to ensure that each measured quantity of the product passes downwards reliably into its bag before the top seal is formed. Examples of stripping devices are described in European patent No.165819 and U.S. Pat. Nos. 2,915,866, 3,070,931, 3,256,673 and 3,262,244. A further example of a stripping device is described in our patent application WO96/32328. The present invention is concerned particularly with new forms of stripping devices for packaging machines of this general type.
According to one aspect of this invention, a packaging machine of the type described, for forming downwardly moving packaging material into bags containing product, comprises a pair of sealing jaws mounted directly or indirectly on parallel shafts for rotation in opposite directions on opposite sides of the path of the packaging material so that the jaws engage the packaging material simultaneously on opposite sides to form successive horizontal seals (or alternate seals if there are, for example, two jaws on each shaft), and stripping means comprising, in association with the sealing jaw on each side of the path of the packaging material, a pair of stripper carriers mounted for rotation around the corresponding shaft at or near the respective ends thereof, or about an axis parallel to the shaft, a horizontal stripper bar mounted at its respective ends on the two stripper carriers and arranged to be movable with respect to the stripper carriers so that the stripper bar can engage the packaging film along a predetermined vertical stripping path along which, in cooperation with a similar stripper bar associated with the sealing jaw on the other side of the packaging material, it acts to displace downwards any product which might otherwise be trapped in the area of the seal, the stripper carriers being arranged to be rotated faster than the shafts at least while the stripper bars are performing each stripping operation.
The stripper carriers are preferably driven by a servo motor separate from the motor driving the shafts, which latter motor may also be a servo motor. In this context by xe2x80x9cservo motorxe2x80x9d we mean a motor of which the output shaft is electronically timed and controlled as to its speed. By this means, the stripper bars can be arranged be driven faster than the sealing jaws while performing each stripping operation; however, if the number of stripper bars equals the number of sealing jaws, for example, then the average speed of the stripper bars would equal that of the sealing jaws. It should be noted, however, that the sealing jaws may be driven at a cyclically varying speed in order to produce the required bag length which may vary for different products; accordingly, as mentioned above, what is important is that the stripper bars should move faster than the sealing jaws so as to overtake them while performing each stripping operation, whatever the speed of the sealing jaws happens to be.
The speed of the jaws, while creating each seal, would normally be equal to the speed of the packaging material which may also be variable as a result of being driven by yet another servo motor.
Instead of the stripper bars being driven by an independent servo motor, they may receive their drive from the motor driving the sealing jaws, for example via a gear train or other mechanism whereby the speed of the stripper bars is arranged to fluctuate cyclically, for example sinusoidally or approximately so, with respect to the speed of the jaws. By this means, a stripper bar could overtake the corresponding sealing jaw to perform each stripping operation, and then slow down to allow the jaw to overtake it in preparation for the next stripping operation. This modification, which constitutes an alternative aspect of this invention, may also be applied to belt or chain mounted stripper bars as described in our patent application WO96/32328.
Alternatively, a common servo motor may be used to drive the sealing jaws and the strippers if, for example, there are two pairs of sealing jaws with a single stripper (on each side of the packaging material) which is driven at a speed exactly twice that of the sealing jaws.
Each stripper bar may be pivotally mounted on its corresponding stripper carriers so that the part of the stripper bar which contacts the packaging material during each stripping operation can move vertically through a predetermined distance, in contact with the packaging material, while overtaking the jaw and performing each stripping operation. One or more springs may be provided to urge the stripper bar into contact with the packaging material with a controlled force. Alternatively, each stripper bar may be controlled as to its angular position with respect to the stripper carriers by means of one or more cams. Another possibility is that each stripper bar may be mounted on its stripper carriers via leaf springs.
According to another aspect of this invention, a packaging machine of the type described comprises a pair of cooperating sealing devices mounted directly or indirectly on parallel shafts for rotation in opposite directions on opposite sides of the path of the packaging material so that the sealing devices engage the packaging material simultaneously on opposite sides to form successive horizontal seals, each sealing device comprising two oppositely disposed sealing jaws whereby two seals are formed during each revolution of the shaft; and stripping means comprising a pair of cooperating stripper bars for engaging opposite sides of the packaging material, each stripper bar being carried at its ends by arms which are pivotally or otherwise movably mounted on rotary stripper carrying members lying beyond the ends of the sealing jaws and being driven at an average or set speed twice that of the shaft, whereby the stripping means strips downwards any product lying within the packaging material, in each region about to be sealed, in advance of each pair of sealing jaws engaging the packaging material, the pivotal or other movement of the arms being such as to allow the stripper bars to move past the sealing jaws in order to perform the stripping operations.
The shaft is preferably driven by a servo motor capable of varying the shaft speed as may be necessary, for example in order to form bags of different lengths. The stripper carriers may be driven by a separate servo motor at an average speed twice that of the shaft; or alternatively, the stripper carriers may be driven by the servo motor driving the shaft but via a transmission which doubles the speed of the stripper carriers relative to the shaft.
The arms carrying each stripper bar may be pivotally mounted on the respective stripper carriers or may alternatively be telescopically mounted so as to slide in a linear bearing or other guide in order to enable the stripper bar to move to different distances from the axis of rotation of the stripper carriers. In either case, the stripper bar is preferably urged outwards by one or more springs, but may alternatively be controlled as to its position by a cam slideway.
According to another aspect of this invention a packaging machine of the type described comprises one or more (preferably a pair) of sealing devices arranged to engage opposite sides of the packaging material tube to produce transverse seals at regular intervals, each sealing device being continuously driven for movement along a closed path; and a pair of stripping devices arranged to engage opposite sides of the packaging material tube to strip downwards any product contained within the tube in the area about to be sealed by the sealing devices; each stripping device comprising a stripper bar carried at its ends by arms which are movably mounted on stripper carriers located near the ends of the corresponding sealing device, whereby movement of the arms relative to the stripper carriers causes or allows each bar to move outwards away from the axis of rotation so as to clear and thus be able to move past the corresponding sealing device so as to perform a stripping operation in advance of engagement of the sealing device with the packaging material, the stripper carrier being driven at a rotational speed such as to cause the stripper to have a speed greater than the sealing device at least while passing the sealing device and performing a stripping operation.